Air-speed indicating system



C- 1948- c. H. TOWNES AIR- SPEED INDICATING SYSTEM Filed July '7. 1944 kwk 51:0 0 0k.

fixation EA m INVENTOR C. H. TOWNES A77 R/VEY Patented Dec. 28, 1948 AIR-SPEED INDICATING SYSTEM 7 Charles H. Townes, Delray Beach, Fla, assignor to Bell Telephone Laboratories, Incorporated, New York,N. Y., a corporation of New York Application July 7, 1944, Serial No. 543,935

This invention. relates to an improved means for obtaining the true air speed of an aircraft in air of known temperature and pressure.

It is the object of this invention to provide .a system of electrical apparatus by means of which an electrical quantity proportional to indicated air speed may be derived and this electrical quantity modified proportionally to known temperature and pressure conditions at the flight altitude to produce an electrical quantity proportional to true air speed.

It is a further object of this invention to provide inan electrical air speed indicating system an electrical element for modifying an electrical quantity proportional to indicated air speed by 4 means of which both temperature and pressure modifications may be introduced by operation of a single control element. 7

A feature of the invention resides in the provision of an electrical system including a source of voltage, a potentiometer and a motor controlled follow-up system associated with the movable element of a Pitot-static tube air speed indicator to derive, by means of the potentiometer, a volt age proportional to the indication of the air speed indicator.

A further feature of the invention resides in the provision of a voltage divider in the electrical system comprising a resistance element which is shaped to give an exponential variation in resistance along its length and with which is provided an altitude scale movable with the resistance tap adjacent a fixed scale representing temperatures, the scales being so calibrated that the registration of the indication representing the known altitude with the indication representing the known temperature will position the resistance tap to introduce the electrical modification requiredto correct the indicated air speed voltage to a voltage corresponding to true air speed less at flight altitude.

The air speed indicator usually employed in aircraft comprises a differential pressure type instrument known as the Pitot-static tube indicator which has connections to thepressure and static tube lines of the aircraft. This instrument gives air speed indications which are correct for the average density at ground level. As the density of the air varies inversely as the temperature and directly as the pressure, a correction must be made to the indicator reading to obtain true air speed at higher altitudes. If the temperature and pressure at the flight altitude are known, true air speed may be determined from the in- 4 Claims. (c1. 73-182) dicated air speed, the relation between them being represented by the equation Vt=true air speed Vi. .indicated air speed T==normal (standard) temperature P|=normal (standard) pressure Tn=air temperature at flight altitude Pn=air pressure at flight altitude K=a fixed quantity whose value is 459.4 for temperatures in degrees Fahrenheit and 2'73 for temperatures in degrees centigrade.

It is present practice to provide a slide rule scale suitably calibrated to permit the navigator or pilot to calculate the true air speed from the air speed as indicated by the Pitot-static tube instrument.

In cases where the true air speed is an important factor which must be determined and correlated with other factors in a. short period of time as, for example, in electrical computers for aircraft bombing, it is desirable to provide a system of apparatus whereby true air speed may be quickly determined and an electrical quantity proportional thereto introduced into the computer with a minimum number of operations. Applicant conceived the idea that the desired condition could be satisfied by a method in which an electrical quantity is automatically derived which is proportional to the indicated air speed, this derived electrical quantity modified in accordance with required corrections for temperature and pressure and the modified quantity introduced directly into the electrical computer.

The system of apparatus developed in accordance with this conception includes means for deriving and constantly maintaining in an electrical circuit to the computer, a voltage proportional to the needle indication of the differential pressure type indicator and means comprising a voltage divider sodesigned that the electrical modifications required for corrections both as to temperature and pressure may be introduced by the manual operation of a single control.-

The invention will be understood from the 'following specification by reference to the accompanying drawings in which:

Fig. l is a diagrammatic view illustrating the system of electrical apparatus for obtaining an electrical indication of ,true air speed in accordance with the invention;

Fig. lhisadiagrammaticviewillustratinga voltage divider having a single resistance element which may be employed in place of the voltage divider bounded by lines X-X, Y--Y in Fig. i.

Fig. 2 is a diagrammatic illustration of the constant impedance voltage divider showing the shape of the resistance elements, their relative angular positionand the relative positions of the wiper arms: and

Fig. 8 shows the calibration and relative position of the temperature and pressure indicator scales used with the voltage divider.

Referring to Fig. l, numeral i indicates the casing of a Pitot-static tube air speed indicator provided with tubular extensions 2 and I to which are connected the aircraft Pitot-static and pressure lines respectively. An indicator needle 4 is movable over a scale 5 calibrated in air speed. The needle 4 is provided with an electrical contact I. A motor driven shaft I is mounted in line with the axis of rotation of meter needle I. An arm I, secured for rotation with shaft I, is provided with an electrical contact 0 which can engage the needle contact 0 through the arc of travel.

Shaft I is driven in either direction by means of a split series reversible motor ill having series field windings Ii and It. The direction of rotation of motor Ill depends on which one of the field windings II and I2 is energized from a suitable source of voltage supplied through lines I: and II. Energization of one or the other of the field windings is governed by the position of relay armature it with respect to contacts It and H which are electrically connected to the motor fleld windings II and I2 respectively. The armature II is biased to engage contact II. Movement of this armature to engagement with contact II is controlled by relay coil it, which is energized by closure of contact 8 on needle 4 and contact 9 on follower arm I.

Anode 23 of a three-element tube 22 and the operating coil It are electrically connected to a suitable source of voltage 24 by wires 25 and 2!. A cathode 28 is connected to ground by wire 28. A heater element is electrically connected to a suitable source of alternating current (not shown). A grid element til is connected through wire It to a battery or other suitable source of voltage 38. The voltage drop across resistance 31 is applied to grid ill. In the position as shown in the drawings, that is, with the contacts 8 and 8 open, the grid 30 is negatively biased to prevent current flow to the anode 23, thus preventing operation of relay coil It, when contact is made between the contacts 6 and 9, an electrical circuit is established from ground through wire ll, contacts't and 8, wire 36 to a point between resistance elements ti and 32, thus short-circuiting resistance 31 to remove the bias from grid til. This operation thus permits current flow to anode 23 and the operating coil.

The arrangement of condensers 40, ti and 42 and resistances 43 and it constitutes a well known expedient for reducing sparking at the contacts It and I1. Limit switches 45 and 48 in circuit to field windings II and I2 respectively, are operable from cams (not shown) rotatably mounted on shaft 1 to limit the rotation of the motor shaft in either direction of rotation.

The operation of the motor controlled followup system is as follows: For the position of armature II as shown in the drawings, the winding I2 is energized to produce rotation of motor ll inadlrectiontomake contactbetween contacts I and I. when this contact is made, relay coil II is energized through tube 22 to move armature ll into engagement with contact It, thus energising iield winding ii to produce rotation of the motor shaft in the opposite direction to break the connection between contacts t and I. On breaking of these contacts the original condition is again set up and the motor will rotate the shaft I in a direction to again make contact between the contactsl and I. 'Ihenetresultis thatthe motor shaft I assumes a position corresponding to the position of the needle 4 of air speed meter I. Relay and motor action are suiilcientiy rapid to cause only a slight vibration of the shaft 1 with negligible load on needle 4.

It will be understood that while a needle contact controlled follow-up system is disclosed, applicant does not intend that his invention be limited to the specific arrangement shown herein as other means for obtaining motor control currents proportional to meter needle deflection may be employed, such as an arrangement of photoelectric cells responsive to needle movement as disclosed in United States Patent 2,167,484,

July 25, 1939, to T. M. Berry or an arrangement of ilxed condenser plates and movable condenser plates attached to the needle as disclosed in United States Patent 1,970,442, August 14, 1934, to B. A. Wittlruhns et al.

In the disclosure in Fla. 1, a potentiometer 50 connected across the source of voltage II is provided with a wiper arm 8| which is secured to motor shaft 1 for rotation therewith. By means of this potentiometer, the wiper arm of which assumes a position corresponding to the position of the air speed indicator needle, a voltage is .derived proportional to the needle indication.

This derived voltage is then. impressed across resistance element 52 and may be read on a voltmeter suitably calibrated in air speeds or fed to an electrical computer over line It.

Between resistance I2 and meter 53 there is interposed a voltage divider including variable resistance elements I and It and fixed resistance element 51. Wiper arms 60 and II are mechanically connected and each is electrically connected to line 54. By means of this mechanical connection the two wiper arms may be moved together in the same direction over resistance elements Ill and It.

The voltage divider disclosed in Figs. 1 and 2 is so designed that for all the necessary adjustments of its shunt and series resistances, the total impedance to the output current remains constant. Theoretically, a voltage divider having one resistance element 55' as shown in Fig. 1A could be employed for fractionallzing the derived voltage when the output is connected to a high impedance circuit. However, as practical considerations dictate that the total impedance to the output current remain constant in the present case wherein the computer is a low impedance circuit, the circuit arrangement employing two variable resistance elements as a constant impedance voltage divider, is employed.

The formula as given above for obtaining true air speed from indicated air speed may be expressed in terms of the ratio of true air speed to indicated air speed, that is, it may be expressed This ratio of speeds is, therefore, expressed in terms of a product of a function of temperature and one of pressure. If this ratio is expressed in terms of the sum of a function of temperature and one of pressure, that is, if it is expressed resistance of element 50 to maintain the constant total impedance condition.

In accordance with this invention a logarithmicaliy graduated scale representing one variable a 6 quantity is made movable with the resistance log 7.- )4 log T. +K log k wiper arm relative to a stationary, logarithmically n l the correct ratio of true air speed voltage to in- $52? fgfig fig iii fiz ig jg f dicated air speed voltage may be O ta in an ment in which the logarithmic pressure scale I2 electrical circuit by resistance wiper arm motion lo having scale indications in terms of altitude is which 15 the sum of Pressure eflect and a attached to the wiper arm shaft 10 for movement perature effect, provided the variable resistance relative to a fix d logarithmically graduated is shaped to give an exponential variation in retemperature scale 13, Sistane a o ts length. Referring to Fig. 3, the shaft 10 has secured If P represents the ratio of the resistance i5 thereto a circular dial 12 which is provided with tween the variable p d around to t to al an altitude scale comprising scale markings from resistance of a resistance element, the ratio 1 of t 30,000 feet, This 11 ,1 1: 1 movable t a Su tab e s ta ce a be expressed P= shaft 10 with the scale indications thereof adwhere 0 is the distance along the resistance elejac nt a fixed member 13 provided with indicament and a and b are constants, the values of n of temperature from .4 t 0 Q which dep nd upon h len h 1 the resistance The distance between each of the indications element and the variation of voltage desired from fr m 0 t 30,000 feet on t t t Scale is one end of the resistance element to the other. t m by b t t t for in the formula This ratio can be expressed as log p=a0+b P which is in turn. equal to log l0g L the values representing the standard pressure of air in millimeters of mercury for each altitude Therefore represented on the scale, the value for P, being T +K P 760; The spacing of the graduations from 0 to g+ g log 30,000 feet altitude for movable scale 12 are $210K Pk shown in the following table:

As it is desired to obtain an exponential variation of voltage ratio with motion of the movable aiming Graduations tap of the resistance element, the size and shape Promo to 30,000 t of resistance elements 55 and 58 are determined by the substitution of suitable values in the for- 1 lag mula p=10" and by the requirement that the 21000- 31000 740 total impedance of the combination of resistance 3: 5:31 elements 55, 56 and 51 be maintained constant. 0,000- 01000 702 The variable resistance element designed in ac- Z1 31 cordance with this formula will therefore give in 01000- 01000 8 4' the circuit shown the desired exponential varia- 1o 32%}; tion of voltage ratio, 111000-121000 816 Referring to Fig. 2, the resistance elements 55 3:32: and 56 are each circularly arranged about a shaft 0-101000 m re 10, to which is secured the resistance wiper arms 3.3g, 60 and GI. This shaft may be rotated by manm ual operation of a control knob I l. J$ 35 3 The wiper arms 60 and iii are moved together n w- .0 by manual rotation of shaft 10. In the position 3 3. as shown in the drawings, the output voltage of fi 1 the voltage divider will produce an air speed read- 00%;833 2% ing on. meter 53 which will be the same as the fi .000 9:16; indicated air speed reading on instrument I. 55 gfi tgw 3.22,

It is generally true that the density of the air 255W" is reduced at altitudes higher than ground level.

i The differential pressure instrument which is de-' The distance between each f the indications pendent on air density will therefore give an air from to 0 on the temperature scale is speed reading at hlgher altitudes which 15 lower 00 obtained bysubstituting for Th in the formula than the true air speed. The voltage derived by means of potentiometer should therefore be a i log 'K modified at higher altitudes in a manner to increase the reading on meter 53. As will be exthe different valuesof temperatures to be repreplained hereinafter, the indicator scales guiding seated on the scale, the-value of T. being 15 C. the movement of shaft 10 are so related and and the constant K being 2'73. The spacing of calibrated that for higher altitudes the shaft 10 is the graduations from 40 to 60 centigrade turned in a direction to reduce the resistance of temperature for fixed scale 13 are shown in. the element while at'the' same time increasing the following table:

The position of the scales in thedrawingsls-ior the average temperature and pressure atground level, that is, zero altitude at 15 C. In the position as shown in Fig, 3, the scale corresponds to the position 01' the wiper arms 60 and 6| as shown in Fig. 2.

It will be obvious from this disclosurelthat by operating the single control knob H to position the altitude dial indication corresponding to known altitude opposite the scaleindication corresponding to known temperature at flight altitude, the voltage divider will produce the variation in voltage ratio required for both temperature and pressure corrections at the flight altitude.

What is claimed is:

1. A system for producing an electrical quantity proportional to true air speed in air of known temperature and pressure comprising in combination an air speed indicating instrument having an indicating element, a source of voltage, means for continuously deriving a voltage from said source proportional to the-movement of the indicating element 01 said instrument, an electrical circuit including said source of voltage, said voltage deriving means and means for m'oditying the derived voltage from saidsource, said last-named means comprising a variable resistance having a variable element, and means for determining the variation of said resistance comprising in combination a stationary scale and a scale movable with the variable element of said resistance adjacent said stationary scale, one 01' said scales being calibrated in terms of temperature and the other of saidscales being calibrated in terms of altitude.

2. A system for producing an electrical quantity proportional to true air speed in air of known temperature and pressure comprising in combination an air speed instrument having an indicating member, a source of voltage, means for continuously deriving a voltage from said source proportional to the positiomot the indicating member of said instrument, an electrical circuit including said source of voltage, said voltage deriving means, and means for modifying the voltage derived from said source proportional to said known temperature and pressure, said last-named means comprising a variable resistance producing an exponential variation in variation in voltage ratio with movement or its adjustable tap, and means determining the movement of said tap comprising a logarithmic temperature scale and a. logarithmic pressure scale,

one of said scales being movable with said resistance tap. I

3. A system for deriving an electrical quantity proportional to true air speed in air of known pressure and temperature and compris ing in combination an air speed instrument having an indicating element, an electrical circuit including a source of voltage, a potentiometer having a movable element and an electrical element' utilizing the voltage from said source, means electrically associated with the indicating element of said instrument and mechanically associated with said potentiometer for positioning the movable element thereoi. in acordance with the position of said instrument indicating element, voltage dividing means in the circuit from said potentiometer including a variable resistance designed to produce an exponential variation in voltage ratio with movement of its movable element, and means associated with the movable element of said variable resistance comprising' a logarithmic temperature scale and a logarithmic pressure scale, one oi said scales being fixed and the other of said scales being movable with the movable element of said resistance.

4. A system for deriving an electrical quantity proportional to true airspeed in air oi! known temperature and pressure comprising in combination an air speed instrument having an indicating element, an electrical circuit including in combination a'source of voltage, means for continuously deriving a voltage from said source proportional to the position of the indicating element of said instrument, means for iractionalizing said derived voltage proportionally to said known temperature and pressure and a utilization circuit connected to said fractionalizing means, said fractionalizing means comprising a voltage divider having an exponentially graded variable resistance in series with said utilization circuit and an exponentially graded variable resistance in shunt with said utilization circuit, a manually operable unitary control means for varying said resistances, and means determining the movement of said control means comprising a stationary, logarithmically graduated temperature scale and a logarithmic pressure scale having scale indications in terms oi. the altitude corresponding to the pressure and movable with said control means adjacent said temperature scale.

CHARLES H. TOWNES.

REFERENCES CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 1,120,214 Metzdorff Dec. 8, 1914 1,573,850 Naiman Feb. 23, 1926 1,847,105 Spitzglass Mar. 1, 1932 1,986,986 Swartout Jan. 8, 1935 2,222,551 Ziebolz et al Nov. 19, 1940 2,318,153 Gilson May 4, 1943 FOREIGN PATENTS Number Country Date 281,527 Germany Jan. 13,1915 482,660 Germany Sept. 20, 1929 5 6,198 Great Britain Dec. 27, 1939 Great Britain Jan. 26, 1940 Certificate of Correction Patent No. 2,457,287. December 28, 1948. CHARLES, H. TOWN ES It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 1, line 40, after the word speed strike out less;

and that the said Letters Patent shouldbe read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 11th day of October, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uammiuioner of Patents. 

